This invention relates to an apparatus and method for reducing interference. More particularly, the invention relates to an interference reduction apparatus and method for reducing interference that arrives via paths from adjacent cells.
Spread-spectrum communication schemes are utilized widely as the most fundamental technique for mobile communications. With a direct sequence (DS) scheme, which is the simplest model of spread-spectrum communication, an information signal of period T to be transmitted is spread-spectrum modulated by being multiplied by a spreading code of chip period Tc (T/Tc=spreading ratio), whereby the spectrum of the signal is spread before the signal is transmitted to the receiving side. On the receiving side the signal that has been spread over a wide frequency range is subjected to despreading to detect the signal component. Despreading is carried out by demodulating the receive signal by multiplying it by a code that is identical with the spreading code.
Many reflected waves in addition to direct waves exist in the signal received on the receiving side. A technique for detecting the correct signal component in such a multipath environment by combining received signals having various different delay-time differences has been established in the form of a RAKE receiver. Specifically, a RAKE receiver, which focuses on the fact that there is an information component contained not only in direct waves but also in delayed waves, despreads the information component, which is present in the delayed waves of each path, based upon the delay timing (path timing) of this path, performs maximal combining upon bringing the timings of the despread signals on respective paths into conformity and outputs the result.
With the conventional RAKE receiver, the timings at which signals arrive via multiple paths are detected by a searcher, each timing is input to a finger corresponding to the particular path, each finger performs despreading at the timing input thereto, and a RAKE combiner combines the despread signals obtained by respective ones of the fingers, thereby demodulating the desired signal.
FIG. 23 is a block diagram of a RAKE receiver according to the prior art. A radio receiver 2 converts a high-frequency signal, which has been received by an antenna 1, to a baseband signal by a frequency conversion, applies quadrature demodulation and inputs the resultant signal to an AD converter 4 via an AGC amplifier 3. The AD converter 4 converts the quadrature-demodulated signal to digital data, and a path searcher 5 calculates the correlation between the received spread-spectrum signal and the spreading code that was used on the transmitting side and searches for the delay timing on each path of the multipath system based upon the result of the correlation calculation. Fingers 6a to 6n are provided in correspondence with respective ones of the paths of the multipath system and each finger has a despreader and a delay circuit, not shown. A timing generator 7 inputs the delay timings of the paths to the despreaders of respective ones of the fingers 6a to 6n as despreading timings, and inputs delay times, which are for bringing into agreement the timings of the despread signals that are output from the despreading units, to the delay circuits of respective ones of the fingers 6a to 6n. The RAKE combiner 8 performs maximal ratio combining of the despread signals that are output from the fingers 6a to 6n and outputs the result to a channel codec (not shown), which is the next stage.
In such a RAKE receiver, each finger performs despreading at the delay timing of the corresponding path. However, signals on other paths at this timing are contained in the despread signal as interference. If the signal components of these other paths, namely the interference components, can be reduced, the BER (Bit Error Rate) can be improved and reception of excellent quality becomes possible.
To achieve this, an interference reduction method referred to as the MIXR (Multipath Interference exchange Reduction) method has been proposed by the present applicant (see the specification of JP 2003-133999A). This interference reduction method reduces interference by using a MICT (Multipath Interference Correlative Timing) signal obtained by performing despreading at the MICT, which is a special timing.
FIGS. 24 to 26 are diagrams useful in describing the MIXR method. Assume that a CDMA mobile station MS receives signals from a base station BTS1 via two paths, as shown in FIG. 24. The signal timings of the two paths received by the CDMA mobile station MS are as shown in FIG. 25. The delay profile of the two paths is assumed to be as shown in FIG. 26. In FIG. 25, A, B, C, . . . , Y, Z are labels representing signals at the timings of respective ones of the paths, in which A is a signal at a correct despreading timing. Let α1, α2 represent channel characteristic values of paths 1, 2, respectively, let t1, t2 represent the despreading timings, and let x1, x2 represent signals obtained by despreading at these timings. Accordingly, if a special timing t0=t1−(t2−t1) is decided, i.e., if a time t0 (=2t1−t2) that is (t2−t1) earlier than t1 is determined and the signal obtained by despreading at this timing t0 is represented by x0, then x1, x2 can be expressed as follows:x1=α1S+α2IZ+n1  (1)x0=α1IZ+α2IY+n0  (2)where α1S represents a desired signal obtained by despreading the receive signal on path 1 from timing t1, α2IZ represents interference obtained by despreading the receive signal on path 2 from timing t1, α1IZ represents a signal obtained by despreading the receive signal on path 1 from timing t0, α2IY represents a signal obtained by despreading the receive signal on path 2 from timing t0, and n1, n0 each represent noise. Although x0 is a signal that is the result of performing despreading at a timing at which the desired signal S is not obtained, α1IZ is contained in x0. In other words, it will be understood that x0 contains a signal having correlation with the interference component α2IZ of x1. In this sense, a signal such as x0 is referred to as a MICS (Multipath Interference Correlative Signal) with respect to paths 1 and 2, and timing such as t0 is referred to as the aforementioned MICT (Multipath Interference Correlative Timing) with respect to paths 1 and 2.
Since x0 has a correlation with the interference component of x1, the interference component of x1 can be diminished by multiplying x0 by a suitable coefficient r and subtracting the product from x1. What this means is that if the coefficient r is decided so as to completely eliminate the interference component IZ contained in x1, the other interference component IY contained in x0 will be increased and, as a consequence, the magnitude of noise overall will increased. The optimum coefficient r is one that is decided in such a manner that the power of overall interference is minimized while leaving the original interference IZ. The coefficient r that will minimize interference components is found as follows:
                                             〈                                                                                                x                    1                                    -                                      rx                    0                                                                              2                        〉                    ⁢                                          =                                                    〈                                                                                                x                      1                                                                            2                                〉                            +                                                                                        r                                                        2                                ⁢                                  〈                                                                                                          x                        0                                                                                    2                                    〉                                            -                              2                ⁢                                  Re                  (                                      〈                                                                  rx                        0                                            ⁢                                              x                        1                        *                                                              〉                                    )                                                      ⁢                                                  =                                                                                〈                                                                                                                    x                          0                                                                                            2                                        〉                                    ⁢                                      (                                                                                                                      r                                                                          2                                            -                                                                        2                          ⁢                                                      Re                            (                                                          〈                                                                                                rx                                  0                                                                ⁢                                                                  x                                  1                                  *                                                                                            〉                                                        )                                                                                                    〈                                                                                                                                                  x                                0                                                                                                                    2                                                    〉                                                                                      )                                                  +                                  〈                                                                                                          x                        1                                                                                    2                                    〉                                            ⁢                                                          =                                                                                          〈                                                                                                                              x                            0                                                                                                    2                                            〉                                        ⁢                                                                                                                    r                          -                                                                                    〈                                                                                                x                                  0                                  *                                                                ⁢                                                                  x                                  1                                                                                            〉                                                                                      〈                                                                                                                                                                      x                                    0                                                                                                                                    2                                                            〉                                                                                                                                                  2                                                        +                                      〈                                                                                                                    x                          1                                                                                            2                                        〉                                    -                                                                                                                                      〈                                                                                    x                              0                              *                                                        ⁢                                                          x                              1                                                                                〉                                                                                            2                                                              〈                                                                                                                              x                            0                                                                                                    2                                            〉                                                                      ⁢                                                                  =                                                                            (                                                                                                                                                                                                α                                1                                                                                                                    2                                                    ⁢                                                      I                            2                                                                          +                                                                                                                                                                          α                                2                                                                                                                    2                                                    ⁢                                                      I                            2                                                                          +                                                  n                          0                          2                                                                    )                                        ⁢                                                                                  ⁢                                                                                  ⁢                                                                                                                    (                                                      r                            -                                                                                                                            α                                  1                                  *                                                                ⁢                                                                  α                                  2                                                                ⁢                                                                  I                                  2                                                                                                                            (                                                                                                                                                                                                                                                                      α                                          1                                                                                                                                                            2                                                                        ⁢                                                                          I                                      2                                                                                                        +                                                                                                                                                                                                                                      α                                          2                                                                                                                                                            2                                                                        ⁢                                                                          I                                      2                                                                                                        +                                                                      n                                    0                                    2                                                                                                  )                                                                                                              )                                                                                            2                                                        +                                                                          ⁢                                                                          ⁢                                                                                                                                    α                          1                                                ⁢                        S                                                                                    2                                    +                                                                                                                                      α                          2                                                                                            2                                        ⁢                                          I                      2                                                        +                                      n                    1                    2                                    -                                                                                                                                                                                                  α                              1                                                        ⁢                                                          α                              2                                                                                                                                2                                            ⁢                                              I                        4                                                                                    (                                                                                                                                                                                                α                                1                                                                                                                    2                                                    ⁢                                                      I                            2                                                                          +                                                                                                                                                                          α                                2                                                                                                                    2                                                    ⁢                                                      I                            2                                                                          +                                                  n                          0                          2                                                                    )                                                                                                                                (          3          )                    where < > represents time averaging.
We write the following:|IZ|2=|IY|2=I2, |n0|2=n02,|n1|2=n12 It will be understood from Equation (3) that the sum of interference and noise will be minimized when the following holds:
                    r        =                                            〈                                                x                  0                  *                                ⁢                                  x                  1                                            〉                                      〈                                                                                      x                    0                                                                    2                            〉                                =                                                    α                1                *                            ⁢                              α                2                                                    (                                                                                                              α                      1                                                                            2                                +                                                                                                α                      2                                                                            2                                +                                                      n                    0                    2                                    /                                      I                    2                                                              )                                                          (        4        )            After reduction, the average value|α2|2I2+n12  (5)of interference and noise before reduction in Equation (1) becomes as follows from Equation (3):
                                                                                      α                2                                                    2                    ⁢                      I            2                          +                  n          1          2                -                                                                                                                  α                    1                                    ⁢                                      α                    2                                                                              2                        ⁢                          I              4                                            (                                                                                                                          α                      1                                                                            2                                ⁢                                  I                  2                                            +                                                                                                              α                      2                                                                            2                                ⁢                                  I                  2                                            +                              n                0                2                                      )                                              (        6        )            and thus it will be understood that the average value of interference and noise is diminished.
Thus, MIXR is a method of reducing overall interference power by replacing part of the interference signal contained in a despread signal with another interference signal.
FIG. 27 is a block diagram of a RAKE receiver that employs the MIXR interference reduction method. A radio receiver (not shown) converts a received high-frequency signal to a baseband signal by a frequency conversion, applies quadrature demodulation and inputs the resultant signal to an AD converter 11. The AD converter 11 converts the input signal to digital data. A path searcher 12 calculates the correlation between the received spread-spectrum signal and the spreading code that was used on the transmitting side and searches for delay timings t1, t2 (see FIG. 26) of respective ones of the paths of the multipath system based upon the result of the correlation calculation. A timing generator 13 for reducing interference calculates timing (2t1−t2) for interference reduction from the delay timings t1, t2, and a despreading timing decision unit 14 inputs the despreading timings t1, t0(t0=2t1−t2) to fingers 150, 151, respectively. The finger 150 performs despreading at the timing t0 and outputs x0 of Equation (2), and the finger 151 performs despreading at the timing t1 and outputs x1 of Equation (1). A synchronous detector/RAKE combiner 16 synchronously detects the despread signals that enter from the fingers, then combines the detected signals upon weighting them (multiplying by the coefficient r) and outputs the resultant signal to a channel codec, which is not shown.
The foregoing is a case where the interference component only on path 1 of channel characteristic α1 is eliminated. However, it can also be so arranged that the interference component on path 2 is eliminated simultaneously and subjected to RAKE combining. Let t1, t2 denote the despreading timings of paths 1, 2, respectively, as shown in FIG. 28, and let x1, x2 represent the signals obtained by despreading at respective ones of these timings. Further, if a special timing t3=t2+(t2−t1)=2t2−t1 is decided, i.e., if a time t3 (=2t2−t1) that is (t2−t1) later than t2 is determined and the signal obtained by despreading at this timing t3 is represented by x3, then x2, x3 can be expressed as follows:x2=α2S+α1IB+n3  (7)x3=α2IB+α1IC+n4  (8)where α2S represents a desired signal obtained by despreading the receive signal on path 2 from timing t2, α1IB represents interference obtained by despreading the receive signal on path 1 from timing t2, α2IB represents a signal obtained by despreading the receive signal on path 2 from timing t3, α1IC represents a signal obtained by despreading the receive signal on path 1 from timing t3, and n3, n4 each represent noise. Although x3 is a signal that is the result of performing despreading at a timing at which the desired signal S is not obtained, α2IB is contained in x3. In other words, it will be understood that x3 contains a signal having correlation with the interference component α1IB of x2. Since x3 has a correlation with the interference component of x2, the interference component of x2 can be diminished by multiplying x3 by a suitable coefficient r′ and subtracting the product from x2.
FIG. 29 is a block diagram of a RAKE receiver for eliminating the interference component contained in path 1 of channel characteristic α1, simultaneously eliminating the interference component contained in path 2 and performing RAKE-combining. A radio receiver (not shown) converts a received high-frequency signal to a baseband signal by a frequency conversion, applies quadrature demodulation and inputs the resultant signal to the AD converter 11. The latter converts the input signal to digital data. The path searcher 12 calculates the correlation between the received spread-spectrum signal and the spreading code that was used on the transmitting side and searches for delay timings t1, t2 (see FIG. 28) of respective ones of the paths of the multipath system based upon the result of the correlation calculation. The timing generator 13 for reducing interference calculates timings (2t1−t2), (2t2−t1) for interference reduction from the delay timings t1, t2, and the despreading timing decision unit 14 inputs the despreading timings t0 (=2t1−t2)t1, t2, t3 (=2t2−t1) to fingers 150, 151, 152, 153, respectively. The finger 150 performs despreading at the timing to and outputs x0 of Equation (2), the finger 151 performs despreading at the timing t, and outputs x1 of Equation (1), the finger 152 performs despreading at the timing t2 and outputs x2 of Equation (7), and the finger 153 performs despreading at the timing t3 and outputs x3 of Equation (8).
The synchronous detector/RAKE combiner 16 synchronously detects the despread signals that enter from the fingers, then combines the detected signals upon weighting them (multiplying by the coefficients r, r′) and outputs the resultant signal to a channel codec, which is not shown.
In a case where multipath within the cell of a base station BTS1 with which communication is currently in progress is the principal factor in interference, i.e., in a case where mobile station MS is near the location of the communicating base station BTS1, the technique using the delay profile within the cell of the communicating base station BTS1 is an effective method of reducing interference, as set forth above.
However, when the mobile station MS moves to the edge of the cell, interference from an adjoining cell becomes a major factor in degradation of characteristics. Even if multipath interference within the cell of the base station with which communication is currently in progress is eliminated, a large reduction in interference is not achieved overall. For example, in a case where one path is viewed from base station BTS1, two paths from base station BTS2 and the mobile station MS is communicating with the base station BTS1, as shown in FIGS. 30 and 31, the interference source is the signal from the adjacent base station BTS2. In such case, interference cannot be eliminated using the methods according to the prior art.